Amazon, which a month ago cut the price of the Kindle to $189, has sold out of the electronic reader and the online retailer gives no indication when the popular device will be back in stock.

On Wednesday, Amazon was still taking orders for the Kindle, but said it would email customers with an estimated delivery date "as soon as we have more information." People who order the device won't be charged until the e-reader ships.

Amazon apparently found the sweet spot when it decided to one-up Barnes & Noble by slashing the price to $10 less than what B&N said it would sell the comparable Nook. Amazon cut the price just hours after B&N dropped the price of the Nook to $199 from $259, which had also been the price of the Kindle.

At less than $200, Amazon is likely spending more to make the Kindle. However, analysts say profits in the digital book market are shifting from the e-reader to the sales of e-books. As a result, lowering prices to get e-readers in the hands of consumers has become the strategy of booksellers.

The strategy is apparently working for Amazon. the company this month announced that unit sales of e-books surpassed that of hardcover books. On Wednesday, Amazon said Swedish author Stieg Larsson has become the first writer to sell more than 1 million digital books on Amazon. Larsson is the author of the bestselling "Millennium Trilogy."

"We've reached a tipping point with the new price of Kindle -- the growth rate of Kindle device unit sales has tripled since we lowered the price from $259 to $189," Jeff Bezos, founder and CEO of Amazon, said at the time. "In addition, even while our hardcover sales continue to grow, the Kindle format has now overtaken the hardcover format."

The growth rate of e-books sales have been impressive, given that digital books overtook hardcover books in less than three years. Amazon has been selling hardcover books for 15 years.

However, given the growing competition in the e-book market, Amazon will have to move quickly to increase its Kindly supply. Besides B&N, Amazon is facing competitive pressure from bookseller Borders, which offers the Kobo e-reader, and from Apple, which has sold more than 3 million iPads.